The balance, including the weight distribution and the airfoil surface variation, of helicopter rotor blades affects the performance and stability of helicopters. Therefore, it is desirable for each of the several rotor blades of a helicopter to have similar weight distributions and similar airfoil surfaces, because having non-uniform rotor blades may impart undesirable vibrations and flight patterns. Helicopter rotor blades often rotate in the range of 200-400 rpm during flight. At these rotational speeds, a small difference in weight distribution and/or airfoil surface contour between rotor blades may have a significant effect on a helicopter's flight performance.
Rotor blades are in the shape of airfoils and may be described in terms of span and chord dimensions. The span of a rotor blade refers to the long, or longitudinal, dimension of the rotor blade, and the chord of a rotor blade refers to the short, or lateral, dimension of the rotor blade. Accordingly, the weight balance of a rotor blade may be described in terms of span-wise weight balance and chord-wise weight balance, with both affecting the overall weight balance and flight of a rotor blade.
Typically, a desired weight distribution, or balance, of a rotor blade is effectuated by appropriate placement of weights within the internal volume of the rotor blade. For example, one or more weights may be positioned selectively along the length of the rotor blade to effectuate a desired span-wise balance of the rotor blade. Additionally, one or more weights may be positioned selectively along the width of the rotor blade, typically at or near the outer tip of the rotor blade, to effectuate a desired chord-wise balance. Additionally or alternatively, rotor blade balance may be described in terms of the position of the center of gravity of the rotor blade, such as in terms of the span and chord dimensions of the rotor blade.
FIG. 1 schematically illustrates a portion of a rotor blade 10 of a BOEING® CH-47 CHINOOK™ helicopter. Specifically, FIG. 1 illustrates a portion of the tip of the rotor blade. The example of FIG. 1 is provided as an illustrative, non-exclusive example and does not limit the scope of the present disclosure. As seen in FIG. 1, the rotor blade defines a fore weight compartment 12 and an aft weight compartment 14. The fore weight compartment includes three rows of cylindrical weights 16 and spacers 18. Each row may be described as a weight package. The three packages are positioned laterally in the chord direction. Placement of the individual weights relative to the individual spacers along the longitudinal length of the packages affects the span balance, whereas placement of the weights relative to the spacers amongst the three packages affects the chord balance. In other words, placement of the weights and spacers in the fore compartment of the illustrated example may affect both the span balance and the chord balance of the rotor blade.
In the illustrated example, the aft weight compartment includes two rows, or packages, of cylindrical weights 16 and spacers 18, with the two packages being positioned vertically with respect to each other, or transverse with respect to the chord direction. Accordingly, placement of the weights relative to the spacers along the longitudinal length of the two packages in the aft weight compartment affects solely the span balance of the illustrated rotor blade. However, weights may be distributed between the fore weight compartment and the aft weight compartment to further adjust the chord balance of the illustrated rotor blade.
FIG. 2 schematically illustrates a portion of a rotor blade 10, including a trim tab 20. As seen in FIG. 2, a trim tab is a piece of hardware attached to the trailing edge 22 of the rotor blade. Typically, a trim tab is a metallic piece of sheet metal that is operatively attached to the rotor blade by sandwiching to outer surfaces of the trailing edge, and extending behind the trailing edge, with the trim tab extending for only a portion of the span-wise length of the rotor blade. The angle of the portion of the trim tab that extends behind the trailing edge relative to the airfoil surfaces of the rotor blade is configured to be selectively adjusted. More specifically, as schematically represented in dashed lines in FIG. 2, a trim tab is configured be selectively angled relative to the airfoil surfaces of the rotor blade to define a desired aerodynamic balance of the rotor blade. An upward angle, or bend, to the trim tab results in a relative downward aerodynamic force being imparted to the rotor blade during rotation. Conversely, a downward angle, or bend, to the trim tab results in a relative upward aerodynamic force being imparted to the rotor blade during rotation.
Traditionally, the weight distribution of helicopter rotor blades may be balanced utilizing static testing and/or dynamic testing. Static testing typically includes placement of a rotor blade on a stand and measuring the weight of the rotor blade at various positions along the span and/or chord of the rotor blade. Such techniques are fairly successful in determining the span-wise weight balance of rotor blades but are less effective in determining the chord-wise weight balance of rotor blades. Moreover, static testing must be performed in a very controlled environment and often takes 2-8 hours or more solely to measure and adjust the weight balance of a single rotor blade.
Dynamic testing is more effective for chord-wise weight balancing of rotor blades and also is used for aerodynamic balancing of rotor blades, but dynamic testing requires the actual spinning of one or more rotor blades together with a master blade. A master blade is a rotor blade that has a desired balance to be matched by the rotor blade(s) being tested. Dynamic testing may be performed utilizing a helicopter itself or by installing a set of rotor blades in a whirl tower. A whirl tower is a controlled environment in which a full set of rotor blades may be installed and observed during rotation. During rotation of a set of rotor blades, including a master blade, the vertical position of the tips of the rotor blades may be observed and compared. A rotor blade that tends to climb, or whose tip is vertically above the tip of the master blade during rotation, may be the result of weight positioned toward the aft of the tip relative to the chord-wise balance of the master blade. Conversely, a rotor blade that tends to dive, or whose tip is vertically below the top of the master blade, is a result of weight positioned toward the fore of the tip relative to the chord-wise balance of the master blade. Additionally, the relative climbing or diving of a rotor blade may be a result of the aerodynamic shape of the airfoil of the rotor blade. Accordingly, based on observation of the relative tip heights of the rotor blades being tested, each rotor blade may be adjusted as desired, such as via the rearrangement, the addition, and/or the removal of weights, as well as via the angular adjustment of the trim tab. Dynamic testing of rotor blades requires trial and error and often requires several (e.g., five or more) flights, or tests, of a single rotor blade simply to adjust the chord-wise weight balance and the aerodynamic balance to a desired result. This amounts to approximately 2-8 hours or more per rotor blade being balanced.
The balancing of rotor blades is important both when they are manufactured as well as when they are serviced after use. Modern rotor blades typically are constructed of composite materials and therefore are highly repairable. That is, composite rotor blades, such as constructed of KEVLAR®, fiber glass, and other fiber and epoxy matrices, tend to be more easily patched than metal rotor blades, for example. However, repaired rotor blades are difficult to balance utilizing existing techniques, because of the high variability of the placement, sizes, and weights of patches and other structures used to repair rotor blades. Moreover, the chord-wise weight balance of a repaired rotor blade may be important not only at the tip of the rotor blade, but at various positions along the length of the span of the rotor blade.